Vertebrate retinas have two types of photoreceptor cells - rods and cones. Rods are exquisitely sensitive to light but cones are more critical for daytime vision, acuity and color discrimination. Cone cells can be adversely affected by genetic mutations and can become secondary casualties of rod degeneration. Environmental factors and aging also can affect cone survival, thereby contributing to age-related macular degeneration. However, little is known about the fine structure of cones due to the paucity of cone cells and lack of proper methods to investigate them has impeded studies of the fine structure. Our long-term objective of this research is to elucidate the detailed structural features of cone photoreceptors critical for their function and survival. Cone phototransduction is initiated by activation of cone pigments (opsins), membrane-bound proteins that are prototypic G protein-coupled receptors (GPCRs). While rhodopsin comprises ~90% of protein in rod disc membranes, the composition and organization of opsins in cone cells have yet to be determined. Such information would be broadly applicable to other signal transduction cascades because GPCRs represent the largest known class of drug, hormone and neuropeptide receptors. Here we propose three thematically linked specific aims related to the structural biology of cone cells: (1) Determine the detailed structure of cone cells in transgenic mice lacking transcriptional factor NRL, a rodent model of enhanced S-cone syndrome in humans. These mice exclusively produce cone-like photoreceptors that will be examined by cryo-electron tomography. The abundance of cone-like cells in this transgenic species will allow creation of protocols required for efficient extraction and imaging studies. (2) Use cryo-electron tomography to elucidate the fine structure of cones in the Nile rat, an animal that has ~33% cone cells compared to ~1% in most other rat species. This study will allow us to discern the native cone structure as compared with rods in the same species. Moreover, the experimental findings in this diurnal rodent should be more generalizable to human photoreceptor ultrastructure and function. (3) Determine the organization of cone pigments in individual disc membranes from both NRL transgenic mice and Nile rats. This will allow us to observe directly if and how the organization of visual pigments differs between rod and cone cells. Information about their organization would improve understanding of human disease states in which rod and cone photoreceptor function is impaired.